Pathways of Transmembrane Electron Transfer in Cytochrome bc Complexes: Dielectric Heterogeneity and Interheme Coulombic Interactions.

The intramembrane cytochrome bc1 complex of the photosynthetic bacterium Rhodobacter capsulatus and the cytochrome b6f complex, which functions in oxygenic photosynthesis, utilize two pairs of b-hemes in a symmetric dimer to accomplish proton-coupled electron transfer. The transmembrane electron transfer pathway in each complex was identified through the novel use of heme Soret band excitonic circular dichroism (CD) spectra, for which the responsible heme-heme interactions were determined from crystal structures. Kinetics of heme reduction and CD amplitude change were measured simultaneously. For bc1, in which the redox potentials of the transmembrane heme pair are separated by 160 mV, heme reduction occurs preferentially to the higher-potential intermonomer heme pair on the electronegative (n) side of the complex. This contrasts with the b6f complex, where the redox potential difference between transmembrane intramonomer p- and n-side hemes is substantially smaller and the n-p pair is preferentially reduced. Limits on the dielectric constant between intramonomer hemes were calculated from the interheme distance and the redox potential difference, ΔEm. The difference in preferred reduction pathway is a consequence of the larger ΔEm between n- and p-side hemes in bc1, which favors the reduction of n-side hemes and cannot be offset by decreased repulsive Coulombic interactions between intramonomer hemes.

Kinetics of charge separation and A0- --> A1 electron transfer in photosystem I reaction centers.

The charge separation P700*A(0) --> P700(+)A(0)(-) and the subsequent electron transfer from the primary to secondary electron acceptor have been studied by subtracting absorption difference profiles for cyanobacterial photosystem I (PS I) complexes with open and closed reaction centers. Samples were excited at 660 nm, which lies toward the blue edge of the core antenna absorption spectrum. The resulting PS I kinetics were analyzed in terms of the relevant P700, P700(+), A(0), and A(0)(-) absorption spectra. In our kinetic model, the radical pair P700(+)A(0)(-) forms with 1.3 ps rise kinetics after creation of electronically excited P700*. The formation of A(1)(-) via electron transfer from A(0)(-) requires approximately 13 ps. The kinetics of the latter step are appreciably faster than previously estimated by other groups (20--50 ps).

Ultrafast primary processes in PS I from Synechocystis sp. PCC 6803: roles of P700 and A(0).

The excitation transport and trapping kinetics of core antenna-reaction center complexes from photosystem I of wild-type Synechocystis sp. PCC 6803 were investigated under annihilation-free conditions in complexes with open and closed reaction centers. For closed reaction centers, the long-component decay-associated spectrum (DAS) from global analysis of absorption difference spectra excited at 660 nm is essentially flat (maximum amplitude <10(-5) absorbance units). For open reaction centers, the long-time spectrum (which exhibits photobleaching maxima at approximately 680 and 700 nm, and an absorbance feature near 690 nm) resembles one previously attributed to (P700(+) - P700). For photosystem I complexes excited at 660 nm with open reaction centers, the equilibration between the bulk antenna and far-red chlorophylls absorbing at wavelengths >700 nm is well described by a single DAS component with lifetime 2.3 ps. For closed reaction centers, two DAS components (2.0 and 6.5 ps) are required to fit the kinetics. The overall trapping time at P700 ( approximately 24 ps) is very nearly the same in either case. Our results support a scenario in which the time constant for the P700 --> A(0) electron transfer is 9-10 ps, whereas the kinetics of the subsequent A(0) --> A(1) electron transfer are still unknown.

Ultrafast primary processes in photosystem I of the cyanobacterium Synechocystis sp. PCC 6803.

Ultrafast primary processes in the trimeric photosystem I core antenna-reaction center complex of the cyanobacterium Synechocystis sp. PCC 6803 have been examined in pump-probe experiments with approximately 100 fs resolution. A global analysis of two-color profiles, excited at 660 nm and probed at 5 nm intervals from 650 to 730 nm, reveals 430 fs kinetics for spectral equilibration among bulk antenna chlorophylls. At least two lifetime components (2.0 and 6.5 ps in our analysis) are required to describe equilibration of bulk chlorophylls with far red-absorbing chlorophylls (>700 nm). Trapping at P700 occurs with 24-ps kinetics. The multiphasic bulk left arrow over right arrow red equilibration kinetics are intriguing, because prior steady-state spectral studies have suggested that the core antenna in Synechocystis sp. contains only one red-absorbing chlorophyll species (C708). The disperse kinetics may arise from inhomogeneous broadening in C708. The one-color optical anisotropy at 680 nm (near the red edge of the bulk antenna) decays with 590 fs kinetics; the corresponding anisotropy at 710 nm shows approximately 3.1 ps kinetics. The latter may signal equilibration among symmetry-equivalent red chlorophylls, bound to different monomers within trimeric photosystem I.

Electronic spectra of PS I mutants: the peripheral subunits do not bind red chlorophylls in Synechocystis sp. PCC 6803.

Steady-state fluorescence and absorption spectra have been obtained in the Qy spectral region (690-780 nm and 600-750 nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium Synechocystis sp. PCC 6803. The 77 K fluorescence spectra of the wild-type and subunit-deficient mutant photosystem I particles are all very similar, peaking at approximately 720 nm with essentially the same excitation spectrum. Because emission from far-red chlorophylls absorbing near 708 nm dominates low-temperature fluorescence in Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room temperature (wild-type-mutant) absorption difference spectra for trimeric mutants lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants are deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 675, and 700 nm, respectively. The absorption difference spectrum for the PsaF/J/I/L-deficient photosystem I complexes at 5 K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance difference spectra (when normalized to the total PS I Qy spectral area) are comparable to the fractions of Chls bound by the respective (groups of) subunits, according to the 4-A density map of PS I from Synechococcus elongatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700 nm.

Excitation delocalization in the bacteriochlorophyll c antenna of the green bacterium Chloroflexus aurantiacus as revealed by ultrafast pump-probe spectroscopy.

Room temperature absorption difference spectra were measured on the femtosecond through picosecond time scales for chlorosomes isolated from the green bacterium Chloroflexus aurantiacus. Anomalously high values of photoinduced absorption changes were revealed in the BChl c Qy transition band. Photoinduced absorption changes at the bleaching peak in the BChl c band were found to be 7-8 times greater than those at the bleaching peak in the BChl a band of the chlorosome. This appears to be the first direct experimental proof of excitation delocalization over many BChl c antenna molecules in the chlorosome.

Energy transfers in the B808-866 antenna from the green bacterium Chloroflexus aurantiacus.

Energy transfers within the B808-866 BChl a antenna in chlorosome-membrane complexes from the green photosynthetic bacterium Chloroflexus aurantiacus were studied in two-color pump-probe experiments at room temperature. The steady-state spectroscopy and protein sequence of the B808-866 complex are reminiscent of well-studied LH2 antennas from purple bacteria. B808-->B866 energy transfers occur with approximately 2 ps kinetics; this is slower by a factor of approximately 2 than B800-->B850 energy transfers in LH2 complexes from Rhodopseudomonas acidophila or Rhodobacter sphaeroides. Anisotropy studies show no evidence for intra-B808 energy transfers before the B808-->B866 step; intra-B866 processes are reflected in 350-550 fs anisotropy decays. Two-color anisotropies under 808 nm excitation suggest the presence of a B808-->B866 channel arising either from direct laser excitation of upper B866 exciton components that overlap the B808 absorption band or from excitation of B866 vibronic bands in nontotally symmetric modes.

Pump-probe anisotropies of Fenna-Matthews-Olson protein trimers from Chlorobium tepidum: a diagnostic for exciton localization?

Exciton calculations on symmetric and asymmetric Fenna-Matthews-Olson (FMO) trimers, combined with absorption difference anisotropy measurements on FMO trimers from the green bacterium Chlorobium tepidum, suggest that real samples exhibit sufficient diagonal energy disorder so that their laser-excited exciton states are noticeably localized. Our observed anisotropies are clearly inconsistent with 21-pigment exciton simulations based on a threefold-symmetric FMO protein. They are more consistent with a 7-pigment model that assumes that the laser-prepared states are localized within a subunit of the trimer. Differential diagonal energy shifts of 50 cm(-1) between symmetry-related pigments in different subunits are large enough to cause sharp localization in the stationary states; these shifts are commensurate with the approximately 95 cm(-1) inhomogeneous linewidth of the lowest exciton levels. Experimental anisotropies (and by implication steady-state linear and circular dichroism) likely arise from statistical averaging over states with widely contrasting values of these observables, in consequence of their sensitivity to diagonal energy disorder.

Ultrafast absorption difference spectra of the Fenna-Matthews-Olson protein at 19 K: experiment and simulations.

We describe simulations of absorption difference spectra in strongly coupled photosynthetic antennas. In the presence of large resonance couplings, distinctive features arise from excited-state absorption transitions between one- and two-exciton levels. We first outline the theory for the heterodimer and for the general N-pigment system, and we demonstrate the transition between the strong and weak coupling regimes. The theory is applied to Fenna-Matthews-Olson (FMO) bacteriochlorophyll a protein trimers from the green photosynthetic bacterium Prosthecochloris aestuarii and then compared with experimental low-temperature absorption difference spectra of FMO trimers from the green bacterium Chlorobium tepidum.

Ultrafast energy transfer in chlorosomes from the green photosynthetic bacterium Chloroflexus aurantiacus.

Energy transfers between the bacteriochlorophyll c and a antennae in light-harvesting chlorosomes from the green bacterium Chloroflexes aurantiacus have been studied in two-color pump-probe experiments with improved sensitivity and wavelength versatility. The BChl c --> BChl a energy transfers are well simulated with biexponential kinetics, with lifetimes of 2-3 and 11 ps. They do not exhibit an appreciable subpicosecond component. In the context of a kinetic model for chlorosomes, these lifetimes suggest that both internal BChl c processes and the BChl c --> BChl a energy-transfer step contribute materially to the empirical rod-to-baseplate energy-transfer kinetics.

Low-temperature energy transfer in FMO trimers from the green photosynthetic bacterium Chlorobium tepidum.

The pump-probe kinetics of the slowest spectral equilibrations between inequivalent BChl a Qy states in FMO trimers from Chlorobium tepidum are decelerated by nearly two orders of magnitude when the temperature is lowered from 300 K to 19 K. The pump-probe anisotropy decays are also markedly slower at 19 K than at 300 K. Singlet-singlet annihilation in FMO trimers is negligible at the laser powers used here. However, reduced temperatures greatly accentuate the probability of singlet-triplet annihilation, due to accumulation of metastable BChl a states under high laser repetition rates.

Femtosecond probe of structural analogies between chlorosomes and bacteriochlorophyll c aggregates.

Bacteriochlorophyll c pigments extracted from light harvesting chlorosomes in green photosynthetic bacteria are known to self-assemble into aggregates whose electronic spectroscopy resembles that of intact chlorosomes. Femtosecond optical experiments reveal that the chlorosomes and their reconstituted aggregates exhibit closely analogous internal energy transfer kinetics and exciton state evolution. These comparisons furnish compelling new evidence that proteins do not exert a major local role in the BChl c antenna pigment organization of intact chlorosomes.

Ultrafast energy transfer in light-harvesting chlorosomes from the green sulfur bacterium Chlorobium tepidum.

Two independent pump-probe techniques were used to study the antenna energy transfer kinetics of intact chlorosomes from the green sulfur bacterium Chlorobium tepidum with femtosecond resolution. The isotropic kinetics revealed by one-color experiments in the BChl c antenna were inhomogeneous with respect to wavelength. Multiexponential analyses of the photobleaching/stimulated emission (PB/SE) decay profiles typically yielded (apart from a approximately 10 fs component that may stem from the initial coherent oscillation) components with lifetimes 1-2 ps and several tens of ps. The largest amplitudes for the latter component occur at 810 nm, the longest wavelength studied. Analyses of most two-color pump-probe profiles with the probe wavelength red-shifted from the pump wavelength yielded no PB/SE rise components. PB/SE components with approximately 1 ps risetime were found in 790 --> 810 and 790 --> 820 nm profiles, in which the probe wavelength is situated well into the BChl a absorption region. A 760 --> 740 nm uphill two-color experiment yielded a PB/SE component with 4-6 ps risetime. Broadband absorption difference spectra of chlorosomes excited at 720 nm (in the blue edge of the 746 nm BChl c Qy band) exhibit approximately 15 nm red-shifting of the PB/SE peak wavelength during the first several hundred fs. Analogous spectra excited at 760 nm (at the red edge) show little dynamic spectral shifting. Our results suggest that inhomogeneous broadening and spectral equilibration play a larger role in the early BChl c antenna kinetics in chlorosomes from C. tepidum than in those from C. aurantiacus, a system studied previously. As in C. aurantiacus, the initial one-color anisotropies r(0) for most BChl c wavelengths are close to 0.4. The corresponding residual anisotropies r(infinity) are typically 0.19-0.25, which is much lower than found in C. aurantiacus (> or = 0.35); the transition moment organization is appreciably less collinear in the BChl c antenna of C. tepidum. However, the final one-color anisotropies at 789 and 801 nm are approximately 0 and 0.09 respectively, and the final anisotropy in time 780 --> 800 nm experiment is approximately -0.1. These facts indicate that the BChI a transition moments themselves exhibit some order, and are directed at an angle > 54.7 degrees on the average from the BChl c moments. The one-color profiles exhibit coherent oscillations at most wavelengths, including 800 nm; Fourier analyses of these oscillations frequently yield components with frequencies 70-80 and 130-140 cm-1.

Femtosecond pump-probe spectroscopy of bacteriochlorophyll a monomers in solution.

One- and two-color absorption difference profiles were obtained for BChl a in 1-propanol with approximately 50-fs resolution, using a self-mode-locked Ti:sapphire laser system. Time evolution in the BChl a absorption difference spectrum produces nonexponential photobleaching/stimulated emission (PB/SE) decay kinetics in 800-nm one-color experiments. Nonexponential PB/SE rise behavior occurs for some combinations of pump and probe wavelengths in two-color experiments. Optimized parameters from triexponential fits to the absorption difference profiles depend markedly on the fitting time window; they typically include a minor component with lifetime in the hundreds of fs. Much of the latter component is due to vibrational relaxation and/or intramolecular vibrational redistribution, rather than solvent dielectric relaxation. Measurements of the pump-probe anisotropy indicate that the electronic transition moment for the broad Qy excited state absorption band that overlaps the Qy steady-state absorption spectrum makes an angle of at most 20 degrees from that of the ground-->Qy state transition. No coherent oscillations are observed at early times. Our results bear directly on the interpretation of fs pump-probe experiments on BChl a-containing pigment-protein complexes.

Ultrafast energy transfer in FMO trimers from the green bacterium Chlorobium tepidum.

Time-resolved absorption difference profiles were obtained for FMO trimers, isolated from the green thermophilic bacterium Chlorobium tepidum, using one- and two-color femtosecond pump-probe techniques. Uphill and downhill energy transfers between inequivalent pigments in this antenna contribute to lifetime components that range from approximately 100 to approximately 900 fs in the isotropic absorption difference signals, depending on the pump and probe wavelengths. Vibrational thermalization of BChl a pigments may also influence the kinetics. The major lifetime components in the anisotropy decays at most wavelengths are 75-135 fs and 1.4-2.0 ps. The slower anisotropy decays probably stem from equilibration among equivalent, lowest-energy pigments belonging to different subunits in the trimer. The initial anisotropy r(0) is appreciably larger than 0.4 at several wavelengths, but r(t) typically decays to a value less than 0.4 within approximately 100 fs.

Low-temperature energy transfer in LHC-II trimers from the Chl a/b light-harvesting antenna of photosystem II.

Temperature dependence in electronic energy transfer steps within light-harvesting antenna trimers from photosystem II was investigated by studying Chl a pump-probe anisotropy decays at several wavelengths from 675 to 682 nm. The anisotropy lifetime is markedly sensitive to temperature at the longest wavelengths (680-682 nm), increasing by factors of 5 to 6 as the trimers are cooled from room temperature to 13 K. The temperature dependence is muted at 677 and 675 nm. This behavior is modeled using simulations of temperature-broadened Chl a absorption and fluorescence spectra in spectral overlap calculations of Förster energy transfer rates. In this model, the 680 nm anisotropy decays are dominated by uphill energy transfers from 680 nm Chl a pigments at the red edge of the LHC-II spectrum; the 675 nm anisotropy decays reflect a statistical average of uphill and downhill energy transfers from 676-nm pigments. The measured temperature dependence is consistent with essentially uncorrelated inhomogeneous broadening of donor and acceptor Chl a pigments.

Femtosecond energy transfer and spectral equilibration in bacteriochlorophyll a--protein antenna trimers from the green bacterium Chlorobium tepidum.

Femtosecond energy transfer processes in a bacteriochlorophyll a-protein antenna complex from the green sulfur bacterium Chlorobium tepidum have been studied by one-color, two-color, and broadband absorption difference spectroscopy. Much of the spectral excitation equilibration in this antenna occurs with 350 to 450 fs kinetics. The anisotropy decay functions r(t) exhibit two major lifetime components, 100 to 130 fs and 1.7 to 2.0 ps. The short component lifetimes may represent single-step energy transfer kinetics in this antenna; the long component is similar to the anisotropy decay observed in earlier picosecond pump-probe experiments.

Ultrafast pump-probe spectroscopy of native etiolated oat phytochrome.

Absorption difference profiles were obtained at wavelengths from 640 to 700 nm with 1-2-ps resolution in a study of primary photoprocesses in the Pr-->Pfr transformation in native oat phytochrome. These experiments were performed using low-intensity laser pulses at high repetition rate; fast sample recycling ensured that essentially all phytochrome species were excited from the Pr ground state. The Pr*-stimulated emission decay at wavelengths > 670 nm exhibits major components with lifetimes of approximately 16 and 50-60 ps. Formation of the asymptotic 695-nm lumi-R absorption spectrum rapidly follows stimulated emission decay. Photoexcitation of one or both of the lumi-R intermediates instantaneously recreates fluorescing Pr* phytochrome, which is spectroscopically and kinetically indistinguishable from that generated by direct illumination of ground-state Pr. This is consistent with assignment of lumi-R as a species in which the chromophore has isomerized from the Z,Z,Z to the Z,Z,E conformation. Anisotropy studies indicate that the orientations of the Pr and lumi-R absorption transition moments are nearly parallel, since little anisotropy decay occurs during the 500-ps time window of these experiments.